Studies of the enzymes of methionine biosynthesis in Lemna revealed the following: (a) O-Acetylserine sulfhydrylase has a 1000-fold higher Vmax and a 70-fold lower Km for sulfide than O-phosphomoserine sulfhydrylase, providing a kinetic basis for the dominance of transsulfurtaion in homocysteine synthesis. (b) Feeding propargylglycine (PAG), a suicide inhibitor of cystathionine gamma-synthase to Lemna showed that only 16% of control synthesis is required to maintain normal methionine synthesis. Therefore, the 85% reduction in synthesis activity observed in the presence of methionine is not sufficient, by itself, to regulate the pathway. (c) Further studies with PAG showed that plants with less than 10% of control cystathionine gamma-synthase activity were not viable in the absence of other factors. Addition of 31 upsilonM L-cystine permitted growth with 9% normal activity probably because internal concentrations of L-cysteine, a cosubstrate for the synthesis, were elevated. (d) L-Cysteine is an irreversible inhibitor of cystathionine gamma-synthase. Whether the inhibition is important in vivo requires further study; plants grown in 31 upsilonM L-cystine have only 60% of the control enzyme activity.